Lower alcohols, such as methanol and ethanol, are important commodity chemicals. For example, lower alcohols, and especially methanol, can be converted to olefins and offer an important route to olefin production that employs non-petroleum sources. In a typical methanol-to-olefin (MTO) reaction, ethylene and propylene are produced in roughly equal yields. However, there are significant economic incentives to produce more ethylene. It has been demonstrated that ethylene selectivity can be significantly improved by co-feeding a small amount of ethanol (5-10 wt. %) with the methanol.
Methanol is commercially produced from synthesis gas (syngas) using copper-based catalysts under forcing conditions (200-300° C. and 50-100 atm). However, due to equilibrium limitations, one-pass conversion is relatively low and so recycle of a large amount of gas is required. Moreover, this process is highly selective to methanol (95+%), with only a small amount of ethanol being formed when high temperatures and low feed rates are employed.
Methanol, the preferred alcohol for light olefin production, is typically synthesized from the catalytic reactions of hydrogen, carbon monoxide and/or carbon dioxide in a methanol reactor in the presence of a heterogeneous catalyst. For example, in one synthesis process methanol is produced using a copper/zinc oxide catalyst in a water-cooled tubular methanol reactor. The preferred process for converting a feedstock containing methanol into one or more olefin(s), primarily ethylene and/or propylene, involves contacting the feedstock with a molecular sieve catalyst composition.
Ethanol is produced commercially either by fermentation or by ethylene hydration catalyzed by an acid catalyst. Alternative routes have been proposed in the literature, including 1) methanol homologation (reaction of methanol with syngas giving ethanol, catalyzed by rhodium, manganese, ruthenium, iron, or cobalt as described in Applied Homogeneous Catalysis with Organometallic Compounds, vol. 2, edited by B. Cornils and W. A. Herrmann, pages 902 to 914, VCH, New York, 1996); and 2) direct synthesis from syngas using supported noble metals such as rhodium as described by Ichikawa (Polyhedron, vol. 7, No. 22/23, pp. 2351-2367, 1988). The homologation route is characterized by harsh reaction conditions (e.g., >300 atm for the Co catalysts), low activity, and low selectivity (the products contain various amounts of acetaldehyde, esters, and even acetic acid). The activities for supported rhodium systems are also low and the products also contain impurities such as acetaldehyde and hydrocarbons.
Direct synthesis of mixed alcohols from syngas has been extensively explored and even tested at pilot scales. Traditionally, the catalysts for mixed alcohol synthesis can be categorized into three groups as discussed in many publications (e.g., P. Forzatti, et al., Catal. Rev.—Sci. Eng., vol. 33 No. 1-2, pp. 109-168, 1991.): 1) mixed metal oxides made via co-precipitation and promoted with alkali metals such as sodium, potassium, rubidium, or cesium; 2) cobalt supported on molybdenum sulfide (Co/MoS2) promoted with potassium; and 3) noble metals supported on an oxide support as discussed in the previous paragraph. However, the driver for mixed alcohol synthesis in the art was to make fuel-grade alcohol mixtures so that they could be blended with gasoline as motor vehicle fuels. Indeed, a wide range of alcohols (C1—C6OH, both linear and branched), significant amounts of paraffins, and even small amounts of olefins are formed from both the mixed metal oxides system and the molybdenum sulfide supported cobalt (Co/MoS2) system and, in many cases, iso-butanol is the major component among the higher alcohols. Iso-butanol is an undesired feed for MTO because it produces predominantly iso-butene, together with oxygenates and coke.
There is a current need for a low-cost route for converting syngas to a methanol/ethanol mixture which contains little or no higher alcohols and which can therefore be fed directly into an MTO process. According to the invention, it has been found that a catalyst comprising a catalytically active metal component on an anionic clay support, such as a hydrotalcite, is effective to convert syngas to a product containing methanol, ethanol, and only small amounts of higher alcohols and other oxygenates.
U.S. Pat. No. 5,472,677 discloses a process for removing N2O from an N2O-containing gaseous mixture by contacting the gaseous mixture with a catalyst produced by heat treating an anionic clay material, such as a hydrotalcite. Example 5 of this patent describes the synthesis of a cobalt-rhodium, aluminum hydrotalcite in which a solution of 1.0 g (10% Rh) rhodium nitrate, 28.81 g cobalt nitrate hexahydrate and 18.76 g of aluminum nitrate nonahydrate in 114 cc of distilled water is added dropwise (over a half-hour period at room temperature) to a 114 cc solution of 14.44 g 97% NaOH and 10.02 g sodium carbonate while maintaining the temperature at or below room temperature. The precipitate is stirred for 2 hours, heated to 65° C. for 18 hours, filtered, washed with large amounts of distilled water to remove excess sodium and nitrate, and dried at 110° C. to produce the desired clay.
U.S. Patent Application Publication No. 2005/0032632, published Feb. 10, 2005, discloses a catalyst composition for use in the conversion of a oxygenated feedstock, such as methanol, into one or more olefin(s), preferably ethylene and/or propylene, wherein the catalyst composition comprises a molecular sieve, such as a silicoaluminophosphate and/or an aluminophosphate, hydrotalcite, and optionally a rare earth metal component, such as lanthanum, yttrium, cerium and mixtures thereof.
U.S. Patent Application Publication No. 2003/0172590, published Sep. 18, 2003, discloses a process for the preparation of synthesis gas from a feedstock containing methane and/or higher hydrocarbons having from about 2 to about 12 carbon atoms by an initial catalytic treatment of the feedstock to provide a methane-containing gaseous mixture substantially free of compounds having 2 or more carbon atoms, and then reforming the gaseous mixture at elevated temperatures using a catalyst obtained by heat treating a nickel-containing hydrotalcite clay.
U.S. Pat. No. 5,653,774 discloses a method for preparing synthesis gas comprising feeding water and a gaseous or vaporizable hydrocarbyl compound to a reaction zone containing a catalyst comprising the composition formed by heat treating under reforming conditions including a temperature of at least 700° C., a catalyst precursor composition comprising at least one hydrotalcite compound having formula:[M2+(1−x)M3+x(OH)2]x+(An−x/n).mH2O,wherein M2+ is a metal ion having a valence of 2+ and is at least Ni2+ ions; M3+ is a metal ion having a valence of 3+; x is a number of about 0.10 to about 0.50; An− is an anion having a negative charge of n; and m is 0 or a positive number. The heat treating converts the hydrotalcite to a new spinel phase; and under reforming conditions the M2+ component is at least partially reduced to produce metal particles of about 1 to about 1000 nanometers in size and containing at least nickel in the zero oxidation state.
U.S. Pat. No. 4,377,643 discloses a process for upgrading syngas to alkanes and alcohols by contacting the syngas with a catalyst comprising mixed oxides of ruthenium, copper, at least one alkali metal and at least one of rhodium, iridium, palladium, and platinum. The catalyst can also include carrier selected from alumina, silica, alumina-silica, alundum, clay, and silicon carbide.
U.S. Pat. No. 4,442,228 discloses a process for the manufacture of ethanol by catalytically reacting carbon monoxide and hydrogen at a temperature in the range of 175° C. to 375° C. and a pressure in the range of 1 to 300 bar in the presence of a supported rhodium catalyst consisting of a rhodium component and at least one co-catalyst selected from the group consisting of zirconium, hafnium, lanthanum, platinum, chromium and mercury wherein said rhodium component and co-catalyst are applied by impregnation onto a catalyst carrier of silicic acid or silicates of elements of Groups II to VIII of the Periodic Table.